The present invention relates to a process for defined impregnation of honeycomb structures with impregnating solutions. The honeycomb structures have flow channels arranged parallel to the main or longitudinal axis of the structure and two cross-sectional surfaces oriented at right angles to the flow channels. The cross-sectional surfaces which form the outer ends in the honeycomb structures may be appropriately shaped for the particular spatial mounting requirements. Circular, square, rectangular and oval cross-sections are most frequently used. Structures that are generally cylindrical are most often used. The flow channels are arranged over the cross-section of the honeycomb structure in a grid which may be designed differently depending on the type of honeycomb structure.
This type of honeycomb structure is used in large numbers in car exhaust gas catalysis. They either consist of correspondingly folded and intertwined metal sheeting or are prepared by extruding ceramic materials. They have between 5 and up to 100 flow channels per cm.sup.2, depending on the actual application. The flow channels are arranged parallel to the direction of flow of the waste gases or exhaust gases to be treated for reduction of emissions. As is well known in the art, these honeycomb structures are coated with a layer of finely divided metal oxides with high surface areas which acts as a support for the catalytically active component. Temperature-stabilized .gamma.-aluminum oxide is generally used for the support layer, which may also contain admixed promoters and base metals to modify the catalytic effect. Transition metal oxides are known for this purpose. Coating the honeycomb structures with these high surface area metal oxides can be performed, for instance, by immersing the honeycomb structure in an aqueous suspension of the finely divided high surface area metal oxides. The thickness of layer which is thereby obtained is between about 10 and 130 .mu.m. Refractory metal oxides are well known in the art for this purpose. The process of depositing these metal oxide coatings is known as the "wash coat" process.
The catalytically active elements are usually noble metals such as platinum, palladium and/or rhodium as is well known in the art. They are either added directly to the aqueous suspension of the high surface area metal oxides, in the form of their water-soluble compounds, such as inorganic salts, or they are applied by impregnation after coating the honeycomb structure with the high surface area metal oxides. In this case, the coated honeycomb structures are immersed in the corresponding noble metal inorganic salt solutions.
It is a common feature of both impregnation procedures that the catalytically active components are applied very uniformly over the cross-section of the honeycomb structure. However, it is desirable to be able to apply the catalytically active components to the honeycomb structure with defined concentration gradients transversely to the direction of flow of the exhaust gas. In fact, it is known from DE 39 12 915 C1 that the concentration of catalytically active components at the edges of the honeycomb structure should be higher than in the middle in order to ensure optimal exhaust gas purification while using the smallest possible amount of catalytically active elements.